1. Field of the Invention
The invention provides a homogenous reaction medium and process for using a homogenous Lewis acid-hydrocarbon complex catalyst for producing an alkylate product that is blendable into motor gasolines.
2. Description of the Related Art
Typical unleaded motor gasolines sold in various octane grades in the United States today are produced by blending together various component streams that are the end products of a variety of hydrocarbon refining processes. For example, a typical gasoline blend may contain, as its components, hydrocracker gasoline produced by catalytic hydrogenation in a "hydrocracking" unit, cracked gasolines produced by a fluidized catalytic cracker, reformate produced by the catalytic reformation of naphtha, isopentane produced by the catalytic isomerization of normal pentane, alkylate produced by the acid alkylation of isobutanes and olefins, normal butane produced from the distillation of crude oil or natural gas, etc. In order to produce gasolines having a specific octane rating and other specific properties, such as vapor pressure, the relative amounts of these component streams in the gasoline blend are adjusted. For example, if it is desired to produce a higher octane rated gasoline, then a larger proportion of the higher octane rated components will be added while lower octane components will be reduced or removed from the blend.
By way of background regarding octane ratings, it has long been recognized that highly branched hydrocarbons and aromatic hydrocarbons, such as benzene, toluene and-xylene, have high octane numbers. This means that when these hydrocarbons are mixed with air under temperature and pressure conditions sufficient to permit complete vaporization, and the mixture is ignited, they burn with a steady rate of combustion and do not burn explosively. Explosive combustion or "knocking" will cause damage to internal combustion engines, if continued for any prolonged period of time. The oil refining industry has developed standards and methods for comparing the combustion of various hydrocarbons and blends of hydrocarbons. 2,2,4 trimethylpentane (commonly called "isooctane") is arbitrarily assigned an octane value of 100 and all other gasoline blending components are compared with this standard.
Alkylates are produced by an acid catalyzed reaction of an alkene with an isoalkane. The alkylate product stream comprises a mixture of multiply branched hydrocarbon compounds of increased carbon number. Highly branched hydrocarbon compounds, such as the trimethyl pentanes, are greatly valued as components for gasoline blends in order to increase the "octane" rating of the gasoline or otherwise modifying other properties of a gasoline fuel.
During the 1930's aircraft of increasing performance required the production of aviation fuels of increased performance, one important property of which was that of a higher octane rating. The highly branched hydrocarbon compounds of a high "octane" rating--such as 2,2,4 trimethylpentane, commonly known as "isooctane," and assigned a 100 octane rating--were not naturally abundant enough in crude oil to be produced in the quantities required for blending with gasoline to meet the quantity demands for high octane aviation fuel.
This gave rise to an intensive study during this early period of methods for producing highly branched alkanes in the gasoline fraction boiling range having high octane properties by reacting lower olefins with lower isoalkanes. There was little economic value in the 1930's-1940's for ethylene or lower isoalkanes, such as isobutane, which further provided incentive to the effort to convert them to highly valuable hydrocarbon products, such as high octane value blending compounds for gasoline fuels.
One method for preparing high octane value hydrocarbons which was developed during this period comprised exposing ethylene and isobutane to an acid-pair composition comprising a metal halide-type Lewis acid and a protic Bronsted acid--most commonly the Lewis acid being AlCl.sub.3 and the protic Bronsted acid being HCl. Under such conditions the ethylene and isobutane react in the presence of the acid pair composition to form multiply branched hydrocarbon compounds of a C.sub.6 -C.sub.8, and higher, carbon number, known as an "alkylate" product.
There are many reports in the literature of the 1930-1960 period on "alkylation" with a Lewis acid-Bronsted acid type of catalyst. See for example R. C. Alden et al., "Diisopropyl" The Oil and Gas Journal, pp. 70-73, 103-107 (Feb. 9, 1946); Clark Holloway et al., "Pilot Plant Production of 2,3-Dimethylbutane", Industrial and Engineering Chemistry, Vol. 38, No. 12, pp. 1231-1238 (Dec. 1946); R. B. Thompson et al., "Production of 2,3-Dimethylbutane by Alkylation", Vol. 40, No. 7, pp. 1265-1269 (July 1948); R. S. Manne, U.S. Pat. No. 2,674,637 (1954); and L. F. Mayhue, U.S. Pat. No. 3,470,264 (1969); and G. F. Prescott et al. U.S. Pat. No. 3,873,635 (1975). As was typical in all such processes, the acid-pair catalyst composition, an aluminumchloride-hydrocarbon complex, formed as a "red oil" or sludge which was not miscible in the ethylene-isoalkane-alkylate hydrocarbon liquid phase. Typically the volume ratio of hydrocarbon feed to red oil catalyst volume ranged from about 1:1 to 1:3 and the reaction had to be performed under vigorous agitation conditions. Further, the activity of the acid-pair catalyst composition eroded over time and as the content of red oil or acid sludge increased. This made it necessary to continuously supply fresh makeup catalyst to the reaction zone while removing then disposing of spent catalyst sludge.
As time progressed, certain events occurred which displaced the acid pair method of alkylate production from commercial use. Non-alkylate octane booster additives were devised--such as the tetraethyl lead of "leaded" gasoline--and other methods were devised for production of alkylate streams, such as by HF and/or H.sub.2 SO.sub.4 acid alkylation. Further, as the polymer industry began to develop and polyethylene came into great demand, ethylene achieved a high product value as a monomer and it became economically undesirable to utilize ethylene for alkylate production.
More recently, with the discovery of the health hazards associated with lead, tetraethyl lead has fallen into disrepute as an octane booster and production of "leaded" gasolines has been banned. This lead to the utilization of aromatic hydrocarbons as octane boosting gasoline additives--such as a combination of benzene-toluene-xylene--for the production of premium unleaded gasoline.
Commercial alkylation plants today may be divided into two categories, those that use sulfuric acid as the catalyst and others that use hydrogen fluoride (HF) as the catalyst for the alkylation reaction. While the sulfuric acid process is the older of the two, the relative importance of the hydrogen fluoride process has increased substantially in recent years so the HF-plants now produce about 47% of all alkylate. By the end of 1990 it was estimated that about 11% of the total gasoline pool consisted of alkylates produced by alkylating isobutane with C.sub.3 -C.sub.5 olefins. Further, alkylation capacity in the United States totaled about 960,000 b/d of alkylate.
Both HF and sulfuric acid alkylation processes are postulated to proceed by the same overall reaction: ##STR1## Thus, a postulated isobutyl cation reacts with an olefin (here 2-butene) to form a branched C.sub.8 cation which in turn reacts with another isobutane molecule to form a neutral C.sub.8 hydrocarbon (2,2,3 trimethylpentane) while regenerating another isobutyl cation. However, a number of competing side reactions also occur, the most troublesome of which produces polymerized olefins ("conjunct polymers" or "tars") which are more soluble in the acid phase than in the isobutane phase. The acid catalyst is not miscible with the hydrocarbon phase ( isobutane/2-butene/alkylate hydrocarbon) and the reaction is accomplished under vigorous agitation. The undesired polymerization reaction proceeds in the acid phase. In contrast, the desired alkylation reaction takes place predominantly at the acid/oil interface. To minimize formation of these polymers or tars and maximize alkylate yield, several operating variables are controlled: the acid:oil ratio in the reactor is minimized; the acid/oil interface is increased by high turbulence; olefin is diluted by a high isobutane: olefin ratio; and reactor temperature is maintained as low as possible.
During the alkylation of isobutane with C.sub.3 -C.sub.5 olefins a portion of the acid catalyst is consumed. For instance, in sulfuric acid catalyzed alkylation, about 0.4-0.6 pounds of sulfuric acid is frequently required to produce about 1 gallon of alkylate, but much lower values, such as 0.1-0.25 pounds per gallon (ppg), can be realized at preferred conditions. Acid costs frequently account for about one third of the total operating costs of sulfuric acid catalyzed alkylation units.
In hydrogen fluoride catalyzed alkylation, HF consumption is often in the range of 0.08-0.25 ppg and regeneration of used HF is relatively easy and cheap. Further, since most of the HF is recovered and recycled, the amount of makeup HF required is small, usually about 0.15-0.2 pounds/bbl of alkylate. However, the conjunct polymers produced with HF alkylation may contain some residual HF and therefore pose a problem of environmentally acceptable disposal.
It now appears that the aromatic hydrocarbon octane boosters produce health hazards, such as being possibly carcinogens, as well as being contributors to ozone formation. The phaseout of aromatics as octane boosters will greatly increase the demand for high octane value alkylates, with their present day method for production giving rise to other concerns.
Aside from the problem of safely disposing of conjunct polymers produced in HF alkylation units, there is also growing public concern about the safety of HF alkylation units. When HF is released into the atmosphere, it forms a fine aerosol which appears to remain at ground level and is then transported by wind. In the event of a release of HF into the air, a concentration in the range of about 2-10 ppm causes irritation of the eyes, skin and nasal passages. Concentrations of about 20 ppm result in immediate danger to life and health. As a result of the hazards posed by inadvertent release of HF from HF alkylation plants, there is a need to develop other technologies for producing alkylate that do not have these attendant risks.
In A. K. Roebuck et al, "Isobutane-Olefin Alkylation With Inhibited Aluminum Chloride Catalyst," Ind. Eng. Chem. Prod. Res. Develop., Vol. 9. No. 1 (March 1970) a renewed focus was given to an aluminum chloride type of catalyst which would dissolve more isobutane, to minimize production of heavy end products compared to HF or H.sub.2 SO.sub.4 alkylation procedures, while also minimizing non-favored by-product as is typical with AlCl.sub.3 based catalyst. An aluminum chloride-ether complex catalyst is described, which in conjunction with various inhibitors, appears capable of giving the desired results under certain conditions. Again, as typical with an aluminum chloride type catalyst, the catalyst phase is not miscible with the hydrocarbon phase, and the reaction medium is a non-homogeneous emulsion produced by vigorous agitation.
There exists a need for a high octane gasoline blending component which is not hazardous to health or the environment to replace aromatic components in the gasoline pool. While this need may be fulfilled by alkylate blending components, there yet exists a need for an alkylation process that is free of the perceived risks to human health and life associated with the use of the HF alkylation process, the tar disposal problem posed by both the sulfuric acid and the HF alkylation processes and the red oil problems associated with the use of an AlCl.sub.3 type catalyst. Further, it is desirable to develop a process of alkylation that utilizes less catalyst in the reactors and held in inventory.